My 3.5 year old has always been a horrible sleeper and really has never slept through the night, at least not consistently.

Lately it has been much worse though and I am really desperate. He is up several times a night between 2-5 times and almost always wakes up the baby (who sleeps in our room - she is 11 weeks).

He is obviously tired during the day bc when he naps he sleeps 3+ hours and I usually end up waking him up, he has also started sleep walking during nap time. We have tried not letting him nap during the day or shorten his naps - but his night sleep is even worse when we do this. Right now he naps about %50 of the days, the other days he has "rest time" but doesn't fall asleep.

Any advice or tips would be greatly appreciated. I am just tired.... and desperate.

My oldest has always had a rough time sleeping. He essentially has no internal clock. When he wakes up in the middle of the night, he thinks it's morning. He came down at 11 one night and started to fix himself cereal. DH (who works late) has caught him a lot lately at 2 doing his morning routine. He's 9 now, so it's easier to tell him to go back to bed and he's quiet when he gets up. It was a lot harder when he was little. I would make sure he came to me first before he did anything when he woke up early. Then if it was the middle of the night I would tell him to "snuggle" and I would get him to go back to sleep with me. When he got too big for that I would put pillows on my floor and let him come sleep on those if he woke up. Then when he hit about 4 I was able to bribe him to stay in his bed all night by a sticker chart and a trip to Chuck E Cheese. I hear there are clocks that you can get that change color when it's safe to get out of your room in the morning. That might be worth trying.

We have a very similar situation, except my boy is 4.5yo. Like your LO, he has always been a bad sleeper. And like you, I am exhausted - I'm now pregnant with n°3, getting up 3 + times to go to the toilets and then getting up to check on my 4.5yo.
I am not sure whether this might be of any help to you, but as for us, we are now looking into child sleep apnea. Sleep apnea is often associated with obese adults, while my LO is a slim and active little boy. But we are fairly sure we are on the right track. Let me know if you want to know more.

My now 7 year old had his tonsils and adenoids out at age 3 due to sleep apnea. Total change around in behavior, attitude, along with a corrected speech impediment caused by the adenoids being so large (effecting his hearing). Than we bought CPAP machine to avoid his bad sleep during the night that is why i would suggest you to check the article https://healthyhowardmd.org/blog/cpa...-what-differs/ to find the answers cause it could help you with your situation. I can only say positive things, but I think most of the reason have alreayd been pointed out!